Well then 20* is too much and will do the opposite; you'll be idling almost exclusively on the mixture screws, and the engine is craving air. Check your mixture screws; the factory setting is 3/4 turn out from lightly seated. Set them there . Then increase the curb idle speed until it runs. Then back the timing up to 12*. Then reset the idle speed to 550 in gear. That will be close, not as exact as taking the carb off and looking. but close. Now check the vacuums again.